A biosensor is a sensor which utilizes the molecule identifying abilities of organic materials such as microorganisms, enzymes, and antibodies, and applies the organic materials as molecular recognition elements. That is, the biosensor utilizes a reaction which occurs when an immobilized organic material recognizes a target specific substance, such as oxygen consumption by respiration of a microorganism, an enzyme reaction, and luminescence.
Among biosensors, enzyme sensors have been put to practical use. For example, enzyme sensors for glucose, lactic acid, cholesterol, lactose, urea, and amino acid are utilized in medical measurement or food industry. An enzyme sensor reduces an electron acceptor by an electron generated by a reaction between a substrate included in a sample solution as a specimen and an enzyme, and a measuring device electrochemically measures the reduction quantity of the electron acceptor, thereby performing quantitative analysis of the specimen. As an example of such biosensor, a sensor proposed in Japanese Published Patent Application No. Hei. 11-324511 has been known.
FIGS. 11 and 12 are exploded perspective views illustrating conventional biosensors for measuring a blood sugar level. A measuring electrode 2 or 102 (also referred to as a working electrode), a counter electrode 3 or 103, and a detecting electrode 4, which electrodes comprise a conductive material, are formed on an insulating support 1 or 101 comprising polyethylene terephthalate or the like, and a reagent layer 5 or 105 including an enzyme which specifically reacts to a specific component in a sample solution, an electron transfer agent, and a hydrophilic polymer is formed on these electrodes.
In order to form a cavity for detecting an electric current value generated by a reaction between the specific component in the sample solution and a reagent in the reagent layer 5 or 105 with the above-mentioned electrodes 2, 3, 4, 102, 103, a spacer 6 or 106 having a spindly cutout part 7 or 107 in a position on the electrodes and the reagent layer, and a cover 8 or 108 having an air vent 9 or 109 are attached onto the insulating support.
In the biosensor constructed as described above, the sample solution is supplied from the inlet of the cavity (sample suction inlet) to the inside of the cavity by capillary phenomenon and is let to the position of the electrodes and the reagent layer. When a specific component in the sample solution reacts to the reagent of the reagent layer, an electric current is generated, and the generated electric current is read by an external measuring device through leads of the biosensor, whereby quantitative analysis of the specimen is carried out.
However, in the biosensor with the above-described reagent composition, under the environment where heat and moisture exist, particularly under the environment of high temperature and humidity where the temperature is over 30° C. and the humidity is over 80%, a reduction reaction occurs between the electron transfer agent and a portion of enzyme protein or hydrophilic polymer which is included in the reagent layer 5 or 105, thereby generating a background electric current (noise electric current). As the value of the background electric current increases with time, the sensor performance is deteriorated.
Furthermore, as a means to solve the problem, it is possible to eliminate moisture and prevent the deterioration of the sensor performance by enclosing a desiccant such as silica gel or activated alumina into a biosensor preservation container which employs a molded container of resin or aluminium seal. However, it is impossible to completely eliminate water of molecular level remaining in the reagent included in the biosensor, with the desiccant alone.
Further, it is extremely hard to keep the preservation container free of moisture penetration over long term, and the reduction reaction between a portion of enzyme protein or hydrophilic polymer and the electron transfer agent is promoted when only a slight amount of moisture exists. Therefore, it is extremely difficult to effectively suppress the increase in the background electric current with time.
Further, when an inorganic salt such as potassium ferricyanide is included in the mixed reagent layer composed of various reagents such as an enzyme and an electron transfer agent, the reagent layer is extremely easily crystallized in the process of drying the reagent solution, whereby the surface of the reagent layer becomes rough and uneven, resulting in deterioration in the response (linearity, sensitivity) of the sensor to the substrate concentration and the measurement accuracy.
The present invention is made to solve the above-mentioned problems and has for its object to provide a highly-accurate biosensor which efficiently prevents deterioration of the performance of the biosensor due to contact with moisture, and has high response (linearity, sensitivity) of the sensor to the substrate concentration.